Modern satellite communication systems provide a pervasive and reliable infrastructure to distribute voice, data, and video signals for global exchange and broadcast of information. Such satellite communication systems also have emerged as infrastructure networks for data communications and entertainment services on-board aircraft. For example, satellite communications networks are now used for broadband services (e.g., Internet access and e-mail and other messaging services) and entertainment (e.g., satellite television and video streaming services) aboard commercial airliners. Further, satellite communications are increasingly used for data communications in other aircraft applications, such as government aircraft applications (e.g., military and first responder aircraft applications), including helicopters.
Additionally, in communications systems, system performance may be aided by employing forward error correction (FEC) or channel coding. Moreover, nearly all such satellite communications systems rely on some form of error control coding for managing errors that may occur due to noise and other factors during transmission of information through the satellite communication channel. Efficient error control schemes implemented at the transmitting end of these communications systems have the capacity to enable the transmission of data (e.g., audio, video, text, etc.) with very low error rates within a given signal-to-noise ratio (SNR) environment. Powerful error control schemes also enable a communications system to achieve target error performance rates in environments with very low SNR, such as in satellite and other wireless systems, where noise is prevalent and high levels of transmission power are costly. More powerful error control schemes, however, result in more complex and costly implementations, if even feasible. Further, in addition to FEC coding, satellite communications systems typically also employ interleaving to improve the performance of the FEC coding.
With respect to helicopters, however, due to physical constraints of helicopter airframes, the signal path between the satellite and the satellite antenna is blocked by the rotary wings, also known as the blades. The period between blockages generally depends on the aircraft design. The duration of the blockages are of a relatively short period of time, depends on a number of parameters, including the width of the blades, the distance between the rotor and the antenna, the azimuth and elevation angle of the satellite, as well as the clearance height between the antenna and the blades. Additionally, the speed of the rotor affects both the periods between blockages and the duration of the blockage. Typically, thermal noise, with Doppler if on a mobile platform, produces the main impairment experienced over the channel for satellite transmissions via a tracking antenna with high directivity. For helicopter-mounted antennas, however, the blockage of the blades adds an additional impairment that dominates transmission performance, overshadowing the effects of thermal noise. Also, multi-paths generated by reflection from the nearest blades and aircraft body can also be an issue, but is generally secondary for highly directed antennas at Ku and Ka band frequencies. The periodic blockage of the blades generally creates two problems. First, receiver synchronization is disrupted by the signal interruption, which can result in loss of synchronization. Loss of synchronization then requires execution of a search and synchronization algorithm to reestablish synchronization. Further, if the next blade blockage occurs prior to reestablishing synchronization, the synchronization algorithm may be further disrupted and/or delayed. Second, data packets or frames transmitted during the period of a blockage is either completely lost or severely attenuated. Accordingly, at the time a blockage begins, and during the duration of the blockage, one or more transmitted data packets will be partially cut-off and/or entirely blocked.
Two prior alternatives are known for addressing such periodic blockage by helicopter blades. A first of these alternatives is to synchronize the data transmissions with the blade rotation. This approach is potentially possible for the return link by monitoring the forward link signal strength to determine the presence of a clear path—that is, if the forward link signal is always transmitted. A problem with this approach is that a latency is involved, and the transmission must be completed before the blockage by the next blade occurs. It is not practical, however, for the network hub to track the blade position of a helicopter on the forward link. Also, with this approach, it is impossible for multiple helicopters to share a single forward link carrier simultaneously, because it is not possible to synchronize individual transmissions to each helicopter, as their blades positions are not synchronized. This technique, therefore, is only useful for the helicopter to hub, or return link, transmissions. The second alternative recovers blocked information through retransmission. Common automatic repeat request (ARQ) retransmission, however, will not work properly, because the blockage can cause an error rate much higher than what is normally expected for ARQ systems to work. Further, the latency for reliable information delivery can be very long due to high retransmission rates. Furthermore, because acknowledgements and repeat requests from the receiving end also have the same blockage issue, a special protocol design taking into account the periodic blockage in both directions is required. A variation of the ARQ technique is to simply repeat the transmission about one half of the blockage period later. In this way, at least one of the data transmissions is assured not to be blocked, but this approach also requires duplicate detection at the receive end to properly reassemble the data stream. Moreover, with this approach, throughput is reduced by less than half, wasting significant bandwidth.
What is needed, therefore, are approaches for data transmissions in a satellite communications system, which accommodate for periodic short duration blockages of the transmission signal to and from a satellite terminal, without packet loss due to the transmission blockages, while employing a robust FEC data recovery schemes.